1. Field of the Invention
The present invention relates to an improved apparatus capable of treating a sludge.
2. Description of the Related Art
A conventional capacity-reducing type sludge treating apparatus is often of such a type that it performs reduction of a sludge capacity by conducting a primary dehydration process by means of a mechanical dehydrator to reduce the water content level of the sludge normally to 80 to 70%, conducting a secondary dehydration process by means of a hydro-extractor and conducting a burning-up process by means of a burn-up furnace.
In this case, as a source of heat to be supplied to the hydro-extractor, use may sometimes be made of heat which recovers by an exhaust heat boiler the exhaust heat of the burn-up furnace. Generally, the calorific value of the sludge is not great enough to recover the exhaust heat to an extent to which all dewatered sludge is dehydrated by a sludge dryer to a low water content level (normally 50-60%) at which spontaneous ignition can occur at the burn-up furnace. It is, therefore, necessary to generate heated vapor at a fuel boiler and supply it to the dryer.
Thus, the operation cost of the sludge treatment depends upon the generation cost of heating vapor. According to the method for supplying the heated vapor via the fuel boiler to the dryer, it is not possible to improve the operation cost.
Conventionally, the composting of the sludge is achieved by dehydrating the sludge by means of the mechanical dehydrator to a water content level of about 70 to 80%, charging it into a fermentation tank to allow it to be aerobically fermented, and stably decomposing organic matter in the sludge into a stable one for use as a compost.
However, the dewatered sludge which is dehydrated by the mechanical dehydrator has its organic matter not effectively decomposed within the fermentation tank in view of its higher water content level. In order to further lower the water content level and to enhance the passage of air in the bulk of the dewatered sludge, additive material, such as chaff and sawdust, is added to the dewatered sludge to allow the sludge to be fermented. For this reason, the cost of the additive material is imposed upon the manufacturing cost of the compost. Furthermore, the additive material, such as the sawdust, if being applied to the irrigated rice paddy, it floated there. In some districts of the world, the stable supply of the above additive material is not necessarily secured, preventing the use of such additive material.
In addition, various germs or bacteria are trapped in the dewatered sludge, failing to effectively ferment the sludge in its natural state. In these circumstances, some of the fermented compost is charged back, as a germ-contained one, into the inlet of the fermentation tank, thus enhancing its fermentation efficiency. Hence the more the dewatered sludge contains the germs or bacteria, the more a resultant compost has to be returned back into the inlet of the fermentation tank. This is the reason why the fermentation efficiency is lowered.
Generally, as a method for drying a sludge, use is made of a dehydrator utilizing heated vapor. Known as this type of dehydrator is a thin-film type dryer which converts a sludge into a thin film under a centrifugal force and, with a heat transfer area thus increased, drys it.
In the conventional dryer of this type, a variation in a liquid sludge to be supplied will result in a variation in the water content level of a dewatered sludge obtained. If there is a momentary change in the amount of sludge generated, the sludge is one stored in a tank at a preceding stage of a drying process and subjected to a batch process, thus facilitating a subsequent burn-up step and burying step. For this reason, a method has been adopted whereby it is possible to obtain a dewatered sludge of a given water content level by the constant supply of a given level of the sludge.
In the aforementioned method, however, a larger storage tank is required at the preceding state and installation spacing is difficult to secure and not economic. The quantity and temperature of heating vapor may be controlled in accordance with a variation in an amount of sludge to be supplied, but a detection response to its variation is not good, and it is also difficult to effect such control.